This invention relates to an improved system for spray dampening the plate cylinder of an offset printing press, and more particularly to a system for more accurately controlling the amount of dampening fluid delivered to the plate cylinder for improvement of print quality.
In the offset printing process, a small amount of a dampening solution, i.e., water with certain additives, is supplied to the offset plate, which then comes in contact with the inking rollers, the ink adhering to the image on the plate and the dampening solution adhering to the other portions of the plate. The quantity and placement of the dampening solution must be varied for different types and densities of ink, variations in printing densities and ink coverages, and press speed. Control of the application of the dampening fluid is particularly important in four-color process, where variations will affect color. If too little fluid is applied, printing will occur in areas where none is desired. If too much fluid is applied, printing may not occur in some areas, and scumming may also occur.
Various systems for dampening the plate cylinder of an offset printing apparatus are in use today. One such system employs dampening rollers which rotate partially within an open trough containing dampening fluid. The dampening rollers bear directly or indirectly against the plate cylinder, thereby supplying a film of dampening fluid to the plate cylinder. This system, however, suffers from a number of inherent disadvantages from the standpoint of both operation and maintenance. From the operational standpoint, the system is too imprecise and difficult to control. Frequently, too much or too little solution is applied to the plate roller, or at least to certain areas of the plate roller, reducing the printing quality. Moreover, since the system is open to the atmosphere, contaminants from paper lint or other foreign material enter the system, algae tends to form in the fluid container, the pH is constantly changing due to evaporation and contamination, and substantial foaming of the fluid tends to occur. In order to reduce these problems to manageable levels, elaborate recirculation systems are commonly employed, and all such systems require constant attention by the press operator. In addition to these problems of control, there is a normal lag time between starting of the printing press and sufficient dampening of the plate cylinder to begin the printing operation. Moreover, there is no means of varying the application of dampening solution to certain areas of the plate cylinder. From the standpoint of maintenance, the cleaning of the dampening system rollers, troughs and other parts is one of the most disagreeable of press room chores.
Another type dampening system which has been employed more recently is the Smith spray system described in U.S. Pat. No. 3,651,756 and No. 3,764,070. This type system eliminates the open fluid container and the immersed dampening roll, and replaces them with a closed system which pumps dampening fluid as a spray onto a dampening roll train for application to the plate cylinder. However, because of the employment of pumps, air is drawn into this system, and therefore the system is prone to the formation of small air bubbles which interfere with the pumping action and causes imprecise application of the dampening fluid. In addition, priming of the system for start-up operation is frequently required. In order to minimize these problems, the system incorporates special priming and air purging apparatus as well as a recirculation circuit which is cumbersome and must be maintained.